Climate Effects of Atmospheric Haze a Little Less... Hazy

September 9, 2009

Scientists have used a new approach to sharpen the understanding of one of the most uncertain of mankind’s influences on climate—the effects of atmospheric “haze,” the tiny airborne particles from pollution, biomass burning, and other sources.

The new observations-based study led by NOAA confirms that the particles (“aerosols”) have the net effect of cooling the planet—in agreement with previous understanding—but arrives at the answer in a completely new way that is more straightforward, and has narrowed the uncertainties of the estimate. The findings appear in this week’s Journal of Geophysical Research - Atmospheres.

The researchers, led by NOAA scientist Daniel M. Murphy of NOAA’s Earth System Research Laboratory in Boulder, Colo., applied fundamental conservation of energy principles to construct a global energy “budget” of the climate’s “credits” and “debits”—heating and cooling processes—since 1950, using only observations and straightforward calculations without the more complicated algorithms of global climate models. They then calculated the cooling effect of the aerosols as the only missing term in the budget, arriving at an estimate of 1.1 watts per square meter. A watt is a unit of power.

The results support the 2007 assessment by the Intergovernmental Panel on Climate Change (IPCC) that estimated aerosol cooling at 1.2 watts per square meter. But the new study places that estimate on more solid ground and rules out the larger cooling effects that were previously thought to be possible.

“The agreement boosts our confidence in both the models and the new approach,” said Murphy. “Plus, we’ve been able to pin down the amount of cooling by aerosols better than ever.”

The narrower bounds on aerosol effects will help in predicting climate change and accounting for climate change to date.

In balancing the budget for the processes perturbing the heating and cooling of the Earth, Murphy and colleagues found that since 1950, the planet released about 20 percent of the warming influence of heat-trapping greenhouse gases to outer space as infrared energy. Volcanic emissions lingering in the stratosphere offset about 20 percent of the heating by bouncing solar radiation back to space before it reached the surface. Cooling from the lower-atmosphere aerosols produced by humans balanced 50 percent of the heating. Only the remaining 10 percent of greenhouse-gas warming actually went into heating the Earth, and almost all of it went into the ocean.

The new study tackled what the IPCC has identified as one of the most uncertain aspects of the human impacts on climate. Aerosols, which can be either solid or liquid, have complex effects on climate. Sulfate particles formed from pollution can cool the Earth directly by reflecting sunlight. Soot from biomass burning absorbs sunlight and warms the Earth. Aerosols can also affect the formation and properties of clouds, altering their influence on climate. The net effect of all these direct and indirect factors is a cooling by aerosols, which has partially offset the warming by greenhouse gases.

Authors of the study are Daniel M. Murphy, Susan Solomon, Robert W. Portmann, and Karen H. Rosenlof of NOAA’s Earth System Research Laboratory; Piers M. Forster of the University Leeds, UK; and Takmeng Wong of the NASA Langley Research Center.

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